Typical color photographic negatives have three records which are sensitive to respective areas of the visible light spectrum, namely red, green and blue. Each record is usually in the form of one or more layers each containing a light sensitive silver halide emulsion. These records also contain couplers which imagewise produce cyan, magenta and yellow dyes, respectively. In a color negative film, the records are usually arranged on a support in the order of red, green and blue sensitive records (that is, the blue sensitive record is furthest from the support).
Conventional silver halide emulsions usually have grains which are primarily cubic, octahedral, cubo-octahedral or polymorphic in shape. Such grains typically have an inherent sensitivity to visible light in the region of about 400-430 nm. Therefore, sensitizing dyes are used on the emulsions to sensitize them to the required red and green region of the spectrum, with a blue sensitizing dye typically being used to sensitize the blue sensitive emulsion to the 450-500 nm region.
Tabular grain emulsions are known for use in the blue sensitive layer of a color negative film. Tabular grains, when present in the blue sensitive layer, result in improved transmission of incident light to the underlying green and red sensitive layers. Such grains are also sensitized in the 450-500 nm region for blue sensitive emulsions. While such grains have little inherent sensitivity in the 400-430 nm range, such emulsions are typically sensitized in the 450-500 nm region since there are more photons in that region than 400-450 nm and thus sensitivity of the blue record is maximized. Since there is a finite amount of grain surface area and hence a limited amount of sensitizing dye that can be adsorbed to silver halide grains, adding additional sensitizing dye to sensitize outside the 450-500 nm region will typically result in less overall sensitivity of the emulsion.
Following imagewise exposure and processing, the image of the negative is usually printed onto a receiver (typically having a paper base although potentially a transparent base might also be used) to yield a positive image. The overall color quality of the prints depends on the relative amounts of cyan, magenta and yellow densities in the negative. Color negative films are designed so that, for a specific taking illuminant (usually daylight), a specified cyan, magenta, and yellow density relationship is effected when a gray uniform target is photographed. However, not all exposed and processed negatives will have a total dye density which in fact integrates over the entire negative to equal gray. There are several causes for this, including chemical processing variations, latent image and film keeping variability, scene spectral illumination variations, as well as scenes composed of objects which do not integrate to gray such as a white cat sleeping on a red car hood.
For example, when pictures are taken under some types of fluorescent lights, prints are usually produced with a green bias which is objectionable. This green bias, or whatever color bias as caused by scene illuminant or other factors described above, can be partially corrected by custom printing the particular negative with the appropriate color filters (that is, by adjusting the amount of red, green or blue light exposure through the negative). In custom printing, such adjustments are made by the person operating the printer, for each negative according to the operator's experience and by trial and error. Custom printing, however, is a time consuming way of producing more acceptable photographic prints.
Automatic printers have been developed to attain rapid and more economical printing from color negatives. Well designed printers have a set of red, green, and blue sensitivities in one large or any number of smaller sensors which are used by the printer algorithum to assess the red, green, and blue densities (that is, the red, green and blue densities integrated by the printer algorithum over the entire negative) in effectively the same way as does a photographic paper which is used in the printer. These printers are set up so that the red, green, and blue densities of a standard negative when exposed with a gray target under the film design illuminant, typically daylight, are recognized as being a neutral film exposure. Thus, for such a negative, the integrated red, green and blue density relative to a gray center, referenced as D', has a value of D'=0. In any printer this leads to adjustment of the appropriate red, green or blue light exposures of the negative to the print (for example, by controlling the duration or intensity of those colors through the use of direct control of the light source(s) and/or filters), to yield a perfect gray print balance.
However, when such an automatic printer encounters an exposed negative for which D' is not equal to zero, the printer algorithm is designed to alter (or "correct") the red, green and/or blue light exposure, in a manner which depends on the value of D'. The degree to which this correction is applied varies depending on the particular printer algorithm used. Due to the diverse causes of color bias, well designed printers do not apply 100% correction. Simple algorithms apply some smaller correction, often 50% to minimize the chances of removing all the color bias in the film which can significantly alter the appearance of captured scenes which do not integrate to gray. More complex algorithms alter the amount of correction depending on the color bias direction(hue) to make a more intelligent assessment as to how much of the bias to correct based on known hue-dependent bias causes. The operation of such algorithms is described in "Modern Exposure Determination for Customizing Photofinishing Printer Response" by E. Goll, D. Hill, and W. Severin, published in Journal of Applied Photographic. Engineering., Vol 5, Number 2, pages 93-104, 1979.
By the foregoing process the automatic printer attempts to remove some or all of the color bias (that is, the degree to which D' differs from 0, sometimes referenced in this application as "saturation" of a negative) recognized by the printer in the film frame. The goal of the printer is to reduce in the print as much as possible, all the color bias in the negative to be printed except that caused by the objects in the scene itself and occasionally some of the bias caused by the scene illuminant (as in pictures taken at sunset) so that the printed reproduction appears to the viewer as the original scene is remembered.
It would be desirable to provide a color negative which can be printed in automatic printers of the above described type and produce prints which have low objectionable color bias even though the negative may have been exposed under different lighting conditions, and particularly under fluorescent lighting. It would further be desirable if such a negative could use a tabular grain emulsion as the silver halide emulsion of the blue sensitive layer.